In commercial inkjet printing systems, a print media is physically transported through the printing system at a high rate of speed. For example, the print media can travel 650-1000 feet per minute. The printheads in commercial inkjet printing systems typically include multiple nozzle plates, with each nozzle plate having precisely spaced and sized nozzles. The cross-track pitch, measured as drops per inch or dpi, is determined by the nozzle spacing. The dpi can be as high as 600, 900, or 1200 dpi. Due to the speed of the moving print media and the high dpi, a reliable system or method is desired for jetting the ink onto the moving print media, for maintaining the alignment of the moving print media with respect to the printheads, and for detecting defects or artifacts in the content printed on the moving print media.
Generally, the streams of drops emitted by each nozzle plate are parallel to each other in order to produce a uniform density on the moving print media. But different failure modes can produce artifacts in the content printed on the moving print media. For example, artifacts are produced by failures in drop deposition or in stitching algorithms that stitch together regions where printheads overlap. These artifacts continue until the problem is corrected. Unfortunately, the necessary corrections may not occur for hundreds or thousands of feet of print media, which results in waste when the printed content is not usable. Additionally, wasted print media causes the print job to be more costly and time consuming.
There are two issues surrounding current artifact or defect detection systems, size and purpose. Current artifact detection systems use cameras configured to image the printed content in a fashion that represents a two-dimensional high resolution scene of the printed content. In order to create a two-dimensional high resolution representation of the printed content, the integration period of the camera is kept relatively short to avoid the blurring associated with longer integration times. Short integration times can be achieved by using a very intense illumination for short bursts that are synchronized with camera integration periods (frequently referred to as strobe illumination), by using a camera with high sensitivity and with short integration periods, or combinations thereof.
One conventional configuration for such cameras is to attach an imaging lens to the camera and then mount the camera to the structure at the distance appropriate to produce a focused image of the print media. The physical configuration of the separate components in the imaging system can consume a large volume of space within or around the printing system. Additionally, it can be difficult to shield the components of the imaging system from the environment created in or around the printing system. Elevated humidity, temperature and a dusty atmosphere can adversely impact the performance of one or more components in the imaging system.
Two-dimensional high resolution imaging of printed content on high speed printers typically requires higher performance cameras and light sources. High resolution imaging can also require the transmission of large amounts of data from the imaging system to a processing device. Due to the amount of data, the processing device requires increasing processing power and time, as well as potentially more complex analysis algorithms, to analyze the data. All of these factors can increase the cost to manufacture and the cost to operate an artifact detection imaging system.
As noted earlier, the other issue with current artifact detection systems is the purpose or product produced by the imaging system. Most commercially available imaging systems are designed to detect discrete artifacts in the printed content, such as impurities or non-uniformities that differ from a nominally uniform background. These non-conforming artifacts can range in size from microns to millimeters. The non-conforming artifacts are randomly dispersed within an otherwise uniform background, which can be wide and moving at a high speed. An example may be a speck of dirt or a strand of hair inadvertently trapped on a paper surface during the manufacturing of a wide roll of paper, and the imaging system is designed to detect these features on a continuous basis. Because these artifacts can be small, the resolution of the camera sensor needs to be sufficiently high to resolve features at the micron level. For example, a 600 dpi resolution imaging sensor can resolve approximately 40 microns, while a 1200 dpi sensor can resolve approximately 20 microns. Higher resolution sensors are usually costlier than lower resolution sensors.
Furthermore, commercially available imaging systems are purposefully designed to avoid blur in the captured image so that the image processing of the captured images can use algorithms to accurately determine the nature of these artifacts. To achieve high resolution, non-blurred images, the imaging systems use high pixel density, two-dimensional (2D) area array sensors capable of a high refresh rate so that large areas of the moving print media can be captured sequentially and continuously. The captured images are then processed to determine the small and randomly occurring artifacts. The larger the digital data set (from the higher resolution sensors or cameras) the more costly the image processing hardware.
High refresh rate systems may also need to use special lighting capable of providing uniform and bright strobe lighting. In order to image large areas across a wide moving print media, captured images from several two-dimensional sensors need to be stitched together or relatively large two-dimensional sensors are required. Given the nominal capability of such high performance imaging systems to meet the needs of the printing industry and the heretofore small number of inkjet printers installed in the industry, there has been little demand for commercial vendors to develop separate imaging systems that can detect printing artifacts that are characteristic of ink jet based printing systems. The cost of printing systems places an exaggerated constraint on the number of imaging systems that can be used with an ink jet printing system, since several such imaging systems may be necessary or beneficial to ensure print quality.